For successful treatment and therapy of diabetes it is important for the patient to use an insulin syringe by means of which he (or she) will be able to inject the required amount of insulin himself. This makes it necessary that the user will be able in a most simple and exact manner to inject the required doses of insulin into the body from an insulin ampulla by means of the syringe.
The different insulin syringes on the market which basically operate very similar to each other, have in common that by means of a manually operated piston the insulin included within a container is urged into a needle and is injected into the patient's body via the stitching end of the needle. All known syringes of this type have in common that they are provided with a throughgoing needle and that accordingly there is a continuous connection between the insulin within the container and the outside air via the needle. This has the result that without any operation by the user insulin rather often drops out of the ampulla and that air flows into the ampulla. If subsequent to the injection the finger pressure is removed from the supply piston, the rubber piston within the ampulla releases very slightly rearwardly, whereby air is sucked into the ampulla through the injection needle. Furthermore, if insulin drops out of or evaporates from the ampulla, air flows into the ampulla. This amount of air, which has entered the insulin ampulla is to be removed by the patient before each injection in order to make sure that, in view of the air flown in, failures in doses are avoided by all means if insulin is injected. Removing this air from the insulin ampulla is tiring and time consuming. Rather often elderly people have difficulties in handling the syringe for removing air.
Furthermore, when injecting insulin, blood vessels often will be pierced. The blood exiting from such vessels subsequent to the injection step when removing the pressure from the piston rod and by means of the rubber piston being slightly released rearwardly within the insulin ampulla is sucked into the ampulla through the through-going needle so that the valuable insulin can no longer be used.